A patch carefully prepared and updated by @ccutrer brings us a byte size option for the batch mode!
See the batch_byte_size option on the Batch class. Both batch_byte_size and batch_size can be used together, whichever is reached first will cause a batch flush. There is no default for batch_byte_size, so no change for existing users unless they opt-in by setting batch_byte_size.
Thank you Cody!